The invention relates to fuel supply and safety systems of processing complexes wherein combustible raw materials are converted to desired products. It is applicable to such complexes as oil refineries and chemical plants wherein gaseous fuel is consumed in process heaters, gas turbines and the like and wherein a separate system of piping serves to conduct gases from pressure relief valves and the like to means to dispose of these rejected gases, usually by incineration. The known incinerators for the purpose include flare stacks, burning pits and the like.
Obviously, the "waste" gases so consumed by incineration are potential fuels but these have not been used for fuel purposes because means were not available to divert them to furnaces and the like without unacceptable compromise with safety and orderly management of the processing complex.
The facilities in a complex such as a petroleum refinery for applying gaseous fuels to useful purpose operate efficiently only when adjusted for consumption of a fuel having a narrow range of properties such as average molecular weight and heating value and free of contaminants such as sulfur compounds, solid particles and components which may condense to liquid phase in the supply lines. The fuel gas system in a petroleum refinery is further maintained at a pressure which will provide, after pressure drop in the supply system, a suitable pressure at the burners of about 15 to 20 pounds per square inch gauge (psig).
By contrast, the pressure relief system is necessarily of a nature to accept large sudden surges of gas to be dumped to the flare upon opening of a pressure relief valve at a process unit. The composition of such surge of gas can vary over wide limits, from streams that are predominantly hydrogen to those which contain relatively heavy, normally liquid hydrocarbons, compounds of sulfur or nitrogen or any of the myriad of other components found in petroleum processing units.
It is common practice to assure safety of the pressure relief system that it be constituted by relatively large pipes sized to cause little back-pressure against flow of sudden large volumes and that the pressure relief system always maintain a positive flow of gas free of oxygen toward the flare or pit. This maintains a flame at the incinerator device for prompt ignition of gases "dumped" by a pressure relief valve and maintains a pressure in the system greater than atmospheric pressure to prevent leakage of air into the system. That result is normally accomplished by bleeding a small amount of fuel gas to the pressure relief system adjacent each pressure relief valve.
In addition to the gas so deliberately introduced, a significant amount of combustible gas enters the pressure relief system from leaking pressure relief valves. Those valves are generally well seated when first installed but can suffer significant leakage in service before being inspected and replaced. For these reasons, the pressure relief system normally conveys a substantial quantity of fuel to a flare or the like for burning without recovery of the heat generated for useful purpose. This disposal of fuel has been accepted as necessary to safety of the refining complex. It is essential that means be provided to conduct for incineration those surges of gas released when a relief valve opens to protect a process unit against excessive pressure arising from some aberration of process conditions. It is also essential that the pressure relief system be under positive pressure of combustible gas to avoid disastrous mixing with air in the system.
Thus, although it is clearly apparent that a fuel is being consumed on a continuous basis by the pressure relief system, acceptable means for recovery of that heating value in whole or part have not been available. Rather, the normal interaction between fuel gas and pressure relief systems has been to provide for discharge as appropriate from the fuel gas system to the flare or other incinerator. See "Petroleum Processing Handbook" Bland & Davidson; McGraw-Hill (1967), FIG. 8-44.